Photomasks are used in photolithographic processes for printing microelectronic circuits and other precision photofabricated parts. In a typical photolithographic process, a substrate is covered with a layer of photoresist material in a desired pattern which is photographically developed by superimposing a photomask over the photoresist material. The photomask has a pattern of opaque and transparent areas with respect to actinic radiation which is passed through the photomask to develop the pattern in the photoresist material. Typically, the actinic radiation is ultraviolet light. The pattern is developed in the photoresist material as a relief image by means of differing solubilities of the exposed and unexposed portions of the photoresist material. Since the preparation of a photomask involves the investment of a substantial amount of time, labor and materials, it is desirable that a photomask be sufficiently durable for repeated use in the manufacture of photofabricated articles. It is also desirable to maximize the resolution of the pattern carried by a photomask in order to improve the precision of the image it transfers to the photofabricated articles.
Some photomasks consist of sheets of glass bearing patterned coatings of photographic emulsion, iron oxide or chromium. Iron oxide and chromium are considerably more durable than photographic emulsions; however, all coating patterned photomasks are subject to scratching and other mechanical damage which shortens their useful life. Photomasks of improved durability comprising a stained pattern within a glass substrate are disclosed in U.S. Pat. No. 3,573,948 to Tarnopol and U.S. Pat. No. 3,732,792 to Tarnopol et al. Although these stained glass photomasks have improved durability, the step of etching a pattern through a stained layer of the glass in the former, or the step of etching through a tin oxide coating in the latter, results in insufficient resolution for some applications.
U.S. Pat. No. 3,561,963 to Kiba discloses a stained glass photomask wherein the desired pattern is etched into a copper film on a glass substrate and copper ions are subsequently migrated into the glass by heating. Although the stained photomask pattern is more durable than a coating, resolution is compromised in this process as a result of the thermal migration step which results in lateral spreading of the stained areas into the adjacent unstained areas.
U.S. Pat. No. 2,927,042 to Hall et al and U.S. Pat. No. 3,620,795 to Kiba disclose methods to minimize the lateral diffusion of staining in the aforementioned processes. The Hall patent describes depositing a film of stain-producing metal onto glass and removing portions of the film by photoetching. An electrical field is then passed through the glass so that the patterned film migrates into the glass substrate. The Kiba patent discloses etching a pattern into a metal film and migrating stain producing ions through apertures in the metal film by heating in an electrical field. Both methods suffer a loss of resolution as a result of the etching step.
U.S. Pat. Nos. 2,732,298 and 2,911,749 to Stookey both disclose the production of a stained image within a glass plate by heating a developed silver-containing photographic emulsion on the glass. However, the use of relatively high temperatures of 400.degree. to 650.degree. C. results in a loss of resolution of the stained pattern.
U.S. Pat. No. 4,155,735 to Ernsberger discloses an improved method for making stained glass photomasks. The method comprises developing a patterned photoresist layer on a glass substrate and then applying an electric field to enhance the migration of staining ions through apertures in the photoresist pattern into the surface of the glass substrate.
In all of the above-described methods for producing patterns in or on photomask substrates, defects can occur in the form of discontinuities in the photomask pattern as a result of dirt on the substrate, or impurities in the photoresist or in the staining material. Since the production of a photomask requires considerable investment of time, labor and materials, it is desirable to repair such defects rather than to reject the photomask plate. The present invention provides a method for repairing such defects in photomask patterns, whether these patterns are in the form of coatings on glass substrates or stained patterns within a glass substrate.